Swedish Pat. No. 390,923 relates to the hot pressing of wood particle board (chipboard) in a hydraulically--operated press having a movable press table and a hydraulic cylinder for moving the press table. Connected to this press is a variable capacity hydraulic pump, which during an initial period of the press cycle is controlled by a control unit so that the hydraulic pressure in the hydraulic cylinder is a function of time. When the wood particle board to be is compressed to a thickness about 1,5 to 2 times its final thickness the control mode is changed from pressure as a function of time to thickness as a function of time. The control unit is switched out, and during a subsequent period of the pressing cycle the variable capacity hydraulic pump--and, thus, the hydraulic pressure in the hydraulic cylinder--will be controlled by the deviation of the actual thickness of the board from the set thickness thereof at each moment of said subsequent period of the pressing cycle. An actual-value transmitter is provided for delivering a signal representing the actual thickness, and a set point transmitter, that has in series a timer, an integrator, and a diode function generator, is provided for delivering a signal representing the desired thickness as a function of time. When the final thickness of the board is reached the press may be cooled to condense steam present in the board in order to prevent the board from bursting from internal steam pressure upon a too fast opening of the press. Alternatively, the press may be opened without cooling if the opening step is carried out slowly enough to permit the steam to escape between the board and the heating platens of the press while the platens prevent the board from expanding noticably.
A suitable control unit for controlling the hydraulic pressure as a function of time is stated to be disclosed in German Democratic Republic (DD) Pat. No. 70,012 and has a template, one edge of which is a graphic representation of the curve p=f (t).
By means of a motor this template is moved past a follower, which over a kinematic system adjusts the positions of two contacts of a contacting pressure gauge that indicates the hydraulic pressure corresponding to the respective position of the follower. The hydraulic pressure will then pulsate between the two values determined by the positions of the contacts.
Such a control system is disadvantageous for several reasons. For example, a system having a variable capacity pump is expensive, the shape of a template is based on guesswork and has to be verified experimentally, and a precise control of the conditions when letting the steam escape between the board and the press platens is almost impossible to obtain.
Other control systems are known from other types of hydraulic presses. For example, in Federal Republic of Germany (DE) Patent Application No. 2,544,794 a press is described that is controlled with regard to end positions, in that the volume of hydraulic fluid supplied to the hydraulic cylinder from an accummulator is accurately measured.
Such a system is not satisfactory for a press for wood-based panels. Further another West-German (DE) Patent Application No. 2,809,387 describes a hydraulic die press or similar press fitted with a similar control device, in which the control valve connected to the hydraulic cylinder is a quick-operating servo valve without fixed positions. This servo valve can be opened, closed or set proportional to a control current in an extremely short time, e.g. approximately two milliseconds.
A quick control like this makes it possible to slow down a punching tool at the instant that it emerges from the blank being pressed. This technical problem is not applicable to presses for wood-based board, and a servo valve like this is complicated and expensive.